This invention relates to an absolute rotational position detection device capable of detecting an absolute value of a rotational position of a motor or other rotating elements in a high resolution of detection.
As an induction type rotational position detector of a type in which primary and secondary windings are provided on a stator side and no winding is provided on a rotor side, known in the art, on one hand, is a rotational type differential transformer of a type which produces an output signal having a voltage level corresponding to a rotational position and, on the other hand, there is a device as disclosed in U.S. Pat. No. 4,604,575 which produces an AC signal having an electrical phase angle corresponding to the rotational position. For improving resolution of detection of such induction type rotational position detection device, it is conceivable to form teeth respectively in a stator and a rotor and cause a permeance change to be produced between the stator and the rotor in such a manner that one pitch of these teeth constitutes one cycle of the permeance change. According to this arrangement, one cycle of change in the output signal corresponds to one pitch of the teeth with resulting improvement in the resolution of detection. This arrangement, however, enables only a relative rotational position within one pitch of the teeth to be detected and so it is insufficient for detecting an absolute rotational position within one rotation. Hence, a second detection device of an inferior resolution which is capable of detecting an absolute rotational position for at least each teeth is required in addition to the detection device provided with the teeth. By providing such second detection device, an absolute value of rotational position can be detected with a high resolution by combining an absolute rotational position for each teeth with a relative rotational angle within one pitch of the teeth. This is disclosed in the above mentioned U.S. Pat. No. 4,604,575. It is apparently expensive, however, to separately construct the toothed detection device of a high resolution and the second detection device of a low resolution and merely arrange these two devices in parallel. There also arises an inconvenience that centering of rotor sections of the two devices must be conducted in high precision. For facilitating this centering, it has been proposed in Japanese Preliminary Utility Model Publication No. 92108/1982 to form the teeth on the rotors in a spiral form to enable a rotational position detection in which one pitch of the teeth constitutes one cycle and a rotational position detection in which one rotation of the spiral constitutes one cycle. This however requires a high-precision machining of the rotors.
It is, therefore, an object of the invention to provide an absolute rotational position detection device capable of detecting an absolute rotational position over the entire circumference of one rotation with a high resolution and yet with a simple construcion.